Programming Exercises

Objectives

• to build and execute a single-qubit circuit

• to display and visualize circuit measurement results

• to explore single-qubit gate operations

• to visualize the single-qubit state

• to explore the concept of quantum superposition

 

Basics ⚙️

1. Importing modules

We need to import Qiskit as well as some other modules and functions into our Python code editor.

Important

You must run this cell each time you reload the page!

import qiskit
import warnings
from qiskit import QuantumCircuit
from qiskit_aer import AerSimulator
from qiskit_aer.primitives import Sampler
from qiskit.quantum_info import Statevector
from qiskit.visualization import plot_bloch_multivector, plot_histogram, visualize_transition

warnings.filterwarnings('ignore', category=DeprecationWarning)

qiskit.__version__   # The version of Qiskit we are using

'1.3.2'

 

2. Creating a quantum circuit with a single qubit

We use the QuantumCircuit() function, and in parentheses we indicate the number of qubits our circuit will have.

qcworkshop = QuantumCircuit(1)  

 

To see what the circuit looks like, we call the draw() function.

qcworkshop.draw(output='mpl')

 

3. Applying a quantum gate

Let’s apply an X gate. This gate changes the state of our qubit to ‘1’ if it is in state ‘0’ and to ‘0’ if it is in state ‘1’.

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What is the classical equivalent of the X-gate?

The quantum X-gate is analogous to the NOT logic gate.

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We call the x() function to apply the gate, but we need to specify the qubit to which the gate is being applied in parentheses. Note that in Python, counting starts from 0 and not from 1.

qcworkshop.x(0)
qcworkshop.draw(output='mpl')

 

4. Measuring a quantum circuit

To measure, a classical bit is added to “collect” the information from the quantum bit. We apply measure_all() to measure every qubit in the circuit.

qcworkshop.measure_all()
qcworkshop.draw(output='mpl')

 

5. Running a quantum circuit

At this point, we have not yet executed the circuit to obtain results. We need to first define the backend on which the circuit is to be runs.

  We will use AerSimulator() to run our job for 2048 shots of the circuit.

my_backend = AerSimulator()                        # defining the backend
my_job = my_backend.run(qcworkshop, shots=2000)    # running the circuit on the backend with shots=2048

And to display the results of our job, we use get_counts() on the result object.

my_results = my_job.result()       # getting the results from job object
counts = my_results.get_counts()   # getting the counts from result object
counts                             # displaying counts

{'1': 2000}

It is noted that the outcome ‘1’ is obtained for all 2000 shots, which is expected.

 

6. Visualizing results

It is nicer if we display the results on a histogram by calling the plot_histogram() function.

plot_histogram(counts, color="coral")         # you can use other colours. For example, try "green" or "blue". 

 

7. Displaying a qubit state

Since we are doing a simulation of the quantum circuit rather than actually executing it on a quantum computer backend, we can display the state without performing a measurement. Here are a few ways to do this.

Printing the statevector

We prepare the same single-qubit circuit as before.

circuit = QuantumCircuit(1)
circuit.x(0)
circuit.draw('mpl')

Next, we read the state of the circuit into a Statevector() object and display it with the draw("latex") option.

circuit_state = Statevector(circuit)
circuit_state.draw("latex")               

\[ |1\rangle\]

Displaying on the Bloch sphere

We can use display the state of a single-qubit on the Bloch sphere using the Statevector object from the previous step. Here’s how it’s done.

plot_bloch_multivector(circuit_state)

We observe that the arrow is pointing towards the South pole of the Bloch sphere, indicating the state \(\ket{1}\).

Note

Note that we needed to recreate a new circuit in order to display the qubit state. This is because obtaining the statevector with a measurement at the end of the circuit would elicit an error message.

 

Exploring single-qubit gates 📈

8. The X-gate

Let us now visualize the action of the X-gate on the state of the qubit. We use a cool function called visualize_transition() for this. This will show a step-by-step transformation of the qubit state from start to finish upon applying the gate.

 

visualize_transition(circuit, trace=True, fpg=30, spg=2)

Once Loop Reflect

 

🧠 CODING ACTIVITY 1 — Apply more than one X-gate

1. Create a one-qubit quantum circuit.

2. Apply two X-gates.

3. Display the circuit state on the Bloch sphere.

4. Next, visualize the transition of the state.

5. Repeat the above exercise for a circuit with three X-gates.

What do you notice?

# WRITE YOUR CODE HERE! 
# 
#
#

 

Answer

The second X-gate undoes the operation of the first and returns the qubit to its initial state \(\ket{0}\). Three X-gates is equivalent to applying a single X-gate with a final state \(\ket{1}\).

two_x = QuantumCircuit(1)
two_x.x(0)
two_x.x(0)
two_x_state = Statevector(two_x)
plot_bloch_multivector(two_x_state) 
#visualize_transition(two_x,trace=True, fpg=30, spg=2)  

 

9. The H-gate

Here’s a more interesting gate. Let’s create a new quantum circuit where we apply a Hadamard gate using h().

h_circuit = QuantumCircuit(1)
h_circuit.h(0)
h_circuit.draw("mpl")

Let’s take a look at the state of the qubit.

h_state = Statevector(h_circuit)
plot_bloch_multivector(h_state)

We’ve just placed our qubit into superposition! Let’s see what happens when we measure our circuit.

h_circuit.measure_all()          # don't forget to add measurement 
h_circuit.draw("mpl")

h_job = my_backend.run(h_circuit, shots=2000) 
h_results = h_job.result()
h_counts = h_results.get_counts()
plot_histogram(h_counts, color="purple")

We can see from the counts that the outcome of 2000 shots is split almost equally between ‘0’ and ‘1’. But why aren’t exactly the same?

 

Answer

It’s because of statistics! When rolling dice, we know that any of the 6 sides is equally likely. However, if we roll only a few times, we may not see a uniform distribution of outcomes. This changes the more we roll the dice.

Try re-running the previous code cell after increasing or decreasing the number of shots.

 

See also

Explore more single-qubit gates in Extras.

 

Multi-Qubit Circuits 🚀

10. Exploring quantum superposition

We saw how the Hadamard gate is able to put a single qubit into superposition. Let’s see the impact of the Hadamard gate with a larger circuit.

big_circuit = QuantumCircuit(3)
big_circuit.h([0,1,2])            # adding an h-gate to qubits 0,1 and 2.
big_circuit.measure_all()
big_circuit.draw("mpl")                            

Ok, let’s run this circuit with a lot of shots.

big_job = my_backend.run(big_circuit, shots=100000) 
big_results = big_job.result()
big_counts = big_results.get_counts()
plot_histogram(big_counts, color="seagreen")

From the histogram, all the outcome states were measured with a fairly uniform distribution. This means that prior to measurement, the state of the circuit is an equal superposition of the states.

 

11. The CX-gate

A key ingredient to quantum computing is performing operations on more than one qubit at a time. This is done with multi-qubit gates.

We will demonstrate here the CX gate. It is defined using a control qubit and a target qubit. If the control qubit is in the state \(\ket{0}\), then nothing is done to the target qubit. If the control qubit is in the state \(\ket{1}\), then an X-gate is applied to the target qubit.

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What is the classical equivalent of the CX-gate?

The quantum CX-gate is analogous to the CNOT logic gate.

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Let’s demonstrate the operation of the CX gate on two qubits. First, we draw the circuit.

cx_circ = QuantumCircuit(2)
cx_circ.cx(0,1)
cx_circ.draw("mpl")

cx_state = Statevector(cx_circ)
cx_state.draw("latex")

\[ |00\rangle\]

 

🧠 CODING ACTIVITY 2 — Demonstrate the operation of a CX-gate

1. Create a two-qubit quantum circuit.

2. Apply an X-gate on the first qubit.

3. Apply a CX-gate which is controlled on the first qubit and targets the second qubit.

4. Draw the statevector.

What do you notice?

# WRITE YOUR CODE HERE! 
# 
#
#

 

Answer

The circuit is now in the state \(\ket{11}\).

circ = QuantumCircuit(2)
circ.x(0)
circ.cx(0,1)
circ_state = Statevector(circ)
circ_state.draw("latex")

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12. Quantum entanglement

Until now, we have not entangled any qubits. This is because entanglement requires a multi-qubit gate. We have learned all the gates we need to learn

 

🧠 CODING ACTIVITY 3 — Demonstrate quantum entanglement

1. Create a two-qubit quantum circuit.

2. Apply an H-gate on the first qubit.

3. Apply a CX-gate which is controlled on the first qubit and targets the second qubit.

4. Draw the statevector.

5. Measure the circuit.

6. Execute the circuit.

7. Plot a histogram of your results.

What do you notice?

# WRITE YOUR CODE HERE! 
# 
#
#

# MORE CODE SPACE
#
#
#

 

Answer

The circuit is now in the state \(\frac{\ket{00}+\ket{11}}{\sqrt{2}}\). The qubits are entangled always to give either the outcome “00” or “11”, where both outcomes are equally probable. The outcomes “01” and “10” are forbidden here.

entangled = QuantumCircuit(2)
entangled.h(0)
entangled.cx(0,1) 

entangled_state = Statevector(entangled)
entangled_state.draw("latex")

---- 

entangled.measure_all()
entangled_job = my_backend.run(entangled, shots=2000)

entangled_results = entangled_job.result()
entangled_counts = entangled_results.get_counts()

plot_histogram(entangled_counts, color="slateblue")

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See also

Explore quantum entanglement further in Extras.